Article
- The EMBO Journal (2008) 27, 1767 - 1778
- doi:10.1038/emboj.2008.104
Published online: 22 May 2008
Subject Categories:
Structural model of the circadian clock KaiB–KaiC complex and mechanism for modulation of KaiC phosphorylation
Rekha Pattanayek1,a, Dewight R Williams2,a, Sabuj Pattanayek1, Tetsuya Mori3, Carl H Johnson3, Phoebe L Stewart2 and Martin Egli1
- Department of Biochemistry, School of Medicine, Vanderbilt University, Nashville, TN, USA
- Department of Molecular Physiology and Biophysics, School of Medicine, Vanderbilt University, Nashville, TN, USA
- Department of Biological Sciences, Vanderbilt University, Nashville, TN, USA
Correspondence to:
Martin Egli, Department of Biochemistry, School of Medicine, Vanderbilt University, 607 Light Hall, Nashville, TN 37232, USA. Tel.: +1 615 343 8070; Fax: +1 615 322 7122; E-mail: martin.egli@vanderbilt.edu
aThese authors contributed equally to this work
Received 5 February 2008; Accepted 28 April 2008
Abstract
The circadian clock of the cyanobacterium Synechococcus elongatus can be reconstituted in vitro by the KaiA, KaiB and KaiC proteins in the presence of ATP. The principal clock component, KaiC, undergoes regular cycles between hyper- and hypo-phosphorylated states with a period of ca. 24 h that is temperature compensated. KaiA enhances KaiC phosphorylation and this enhancement is antagonized by KaiB. Throughout the cycle Kai proteins interact in a dynamic manner to form complexes of different composition. We present a three-dimensional model of the S. elongatus KaiB–KaiC complex based on X-ray crystallography, negative-stain and cryo-electron microscopy, native gel electrophoresis and modelling techniques. We provide experimental evidence that KaiB dimers interact with KaiC from the same side as KaiA and for a conformational rearrangement of the C-terminal regions of KaiC subunits. The enlarged central channel and thus KaiC subunit separation in the C-terminal ring of the hexamer is consistent with KaiC subunit exchange during the dephosphorylation phase. The proposed binding mode of KaiB explains the observation of simultaneous binding of KaiA and KaiB to KaiC, and provides insight into the mechanism of KaiB's antagonism of KaiA.
Keywords:
- electron microscopy,
- image reconstruction,
- protein–protein interactions,
- X-ray crystallography
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